Analysis of the Inhibition Layer of Galvanized Dual-Phase Steels

The formation of the Fe-Al inhibition layer in hot-dip galvanizing is a confusing issue for a long time. This study presents a characterization result on the inhibition layer formed on C-Mn-Cr and C-Mn-Si dual-phase steels after a short time galvanizing. The samples were annealed at $800^{\circ}C$ for 60 s in $N_{2}$-10% $H_{2}$ atmosphere with a dew point of $-30^{\circ}C$, and were then galvanized in a bath containing 0.2 %Al. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) was employed for characterization. The TEM electron diffraction shows that only $Fe_{2}Al_{5}$ intermetallic phase was formed. No orientation relationship between the $Fe_{2}Al_{5}$ phase and the steel substrate could be identified. Two peaks of Al 2p photoelectrons, one from metallic aluminum and the other from $Al^{3+}$ ions, were detected in the inhibition layer, indicating that the layer is in fact a mixture of $Fe_{2}Al_{5}$ and $Al_{2}O_{3}$. TEM/EDS analysis verifies the existence of $Al_{2}O_{3}$ in the boundaries of $Fe_{2}Al_{5}$ grains. The nucleation of $Fe_{2}Al_{5}$ and the reduction of the surface oxide probably proceeded concurrently on galvanizing, and the residual oxides prohibited the heteroepitaxial growth of $Fe_{2}Al_{5}$.